1 Field of the Invention
The present invention generally relates to a display device, and more particularly to a display incorporating a holographic optical element.
2. Description of Background Art
In our earlier application WO93/02372 we describe a display device incorporating a holographic optical element. Light incident on the holographic optical element is directed to a single viewing zone. By moving the source of the light, the position of the viewing zone can be moved. Temporally alternating left and right images are projected on the screen from alternating positions whereby the images are viewable one after the other in respective left and right hand viewing zones.
According to a first aspect of the present invention, we provide a display device comprising a holographic optical element having at least two sets of hologram regions, the region(s) of the first set being interleaved or overlapping with adjacent region(s) of the second set(s) and being constructed such that light incident on each set of regions is diffracted so as to construct a respective one or more of a plurality of real or virtual images of a diffuse light source; and image generating means comprising a plurality of image elements; wherein the holographic element and its hologram regions are disposed and designed such that light diffracted by the first set of hologram regions passes through a corresponding first set of image elements and such that light diffracted by the second set of hologram regions passes through a corresponding second set of image elements.
According to a second aspect of the present invention, we provide a display device comprising a holographic optical element having at least two sets of hologram regions, the region(s) of the first set being interleaved or overlapping with adjacent region(s) of the second set(s) and being constructed such that light incident on each set of regions is diffracted so as to construct a respective one or more of a plurality of real or virtual images of a diffuse light source; and image generating means comprising a plurality of image elements; wherein the holographic optical element and its hologram regions are disposed and designed such that light having passed through a first set of image elements of the image generating means is diffracted by the first set of hologram regions and such that light having passed through a second set of image elements of the image generating means is diffracted by the second set of hologram regions.
Each set may comprise a single region, which extends across a large proportion of the holographic optical element. Typically however, each set of regions comprises an array of regions.
Typically the regions making up an array are laterally offset from each other (e.g. vertically and/or horizontally).
Each region in an array is typically spaced from and does not overlap with adjacent regions in the array, although adjacent regions in the same array may overlap.
Also, each set of regions is typically laterally offset from the other set(s) of regions. The regions within an array may overlap with regions of other arrays. Typically however, the regions of each array do not overlap with regions of other arrays.
The at least two sets of regions may make up a lateral array of vertical or horizontal stripes, or a two-dimensional array such as a honeycomb.
In a preferable embodiment, the holographic optical element comprises two interleaved sets of regions.
Typically the holographic optical element is constructed such that each set of regions comprises a holographic recording of one or more diffuse light sources.
By making a recording of the same light source, or a number of different light sources in a plurality of different positions with respect to the holographic optical element, the light incident on the holographic optical element is directed to a position corresponding with the position(s) of the diffuse light source(s). Put another way, when each of the sets of regions is illuminated with light, the diffracted light forms an image of the one or more diffuse light sources. Typically the image is a real image although it may also be a virtual image.
Where each set of regions comprises a recording of only one diffuse source, then light incident on the set of regions will be directed to a single viewing zone. Where each set of regions comprises a recording of a plurality of diffuse sources, then light incident on the set of regions will be directed to a plurality of viewing zones.
The holographic optical element is typically a transmissive element, although it may be reflective.
The holographic optical element may be formed in any suitable way, for instance as a surface relief hologram made e.g. by moulding or embossing, or as a volume hologram made in e.g. photopolymer, dichromated gelatine or silver halide.
Typically the holographic optical element is provided as part of a display device such as a stereoscopic display device. This provides a particularly compact and easily constructed spatially multiplexed two or three-dimensional display device with holographically generated viewing apertures and capable of the display of images in real time.
Typically the display device is provided with image generating means defining a plurality of images each corresponding to a respective array of image elements (such as an array of LCD pixels). Typically the holographic optical element is also recorded with a holographic recording of an array of apertures. Where the element is incorporated in a display device, the array of apertures is arranged to coincide with arrays of alternating image elements in the display. By arranging the holographic optical element and the array of image elements in this way, light passing through the image elements is diffracted to a respective viewing zone by one of the sets of regions in the holographic optical element.
In a first example, the holographic element is placed adjacent to and in front (i.e. the viewer""s side) of the LCD.
The LCD may be backlit from a single source. Alternatively the LCD may be backlit from a number of sources grouped so as to be functionally equivalent to a single source.
The source or sources may be moved in order to move the positions of viewing zones. Alternatively the source may be static and its optical image moves (e.g. through the use of moving mirrors or a variety of other means). In a further alternative an array of static sources or a single elongated source with a controllable masking element may be provided so that the effective position of the source is moved by switching lights on/off or moving the masking element respectively.
Typically the LCD display carries a plurality of interleaved images (for instance in the form of a set of lateral stripes or in a honeycomb array).
The plurality of images may comprise the same view of a two-dimensional image if a two-dimensional display is required. In this case the source is preferably a plurality of laterally offset sources or a linear sourcexe2x80x94so as to overlap the left and right viewing zones thereby providing a full resolution 2D image to both eyes.
Typically however, the plurality of images comprise left and right-hand stereoscopic views. The regions of the holographic optical element typically correspond with the array of LCD image elements. The holographic regions corresponding to the left-hand image elements diffract the light to (and hence render the image visible from) a left-hand viewing zone and vice-versa for the right-hand image.
Alternatively the holographic optical element may be placed behind the LCD. In this case, the holographic optical element may be placed adjacent to the LCD, in which case the sets of regions will coincide with the array of the LCDs. However, if the holographic optical element is spaced from the LCD, then the regions of each set will typically overlap with regions from the other set, or even with regions of their own sets. In a limiting case, each set of regions may comprise a single region extending across the holographic optical element. However, the holographic optical element is still recorded and configured such that light, which passes through e.g. the left-hand LCD image elements, is only directed to the left hand viewing zone, and vice-versa.
The holographic optical element may be constructed in a number of ways as described more fully in the following specification. However, common features of the recording method are that, at some stage in the recording, an image of a diffuse light source is recorded with the light source in a plurality of positions with respect to the holographic plate, each position relating to a respective viewing zone.
The holographic optical element may be recorded in a xe2x80x9cone stepxe2x80x9d process in which a diffuse light source is recorded in a plurality of positions in two exposures.
Typically however, the holographic optical element is recorded as a transfer hologram from one or more master hologram plates.
Another common feature of the holographic recording process is that at some stage in the process, a mask (corresponding to the array of LCD image elements in the display device) is used in the recording process. The mask may be placed adjacent to the holographic plate in the recording of either the master hologram or the transfer hologram. Alternatively, the mask may be placed a distance from the holographic plate in which case it can be considered that a holographic recording is being made of the viewing apertures created by the mask.
The holographic optical element may be constructed as a substrate guided wave device.
In addition, the display device may further comprise a focusing optic, which creates a virtual image of the array of image elements and the holographic optical element and forms real images of the viewing zones. In such a case the image of the diffuse light source produced by the holographic optical element might advantageously be formed as a virtual image in the first instance.